Added index for url target (to improve performance later on)

migrations/20210812194727_init_db.js

@@ -9,7 +9,7 @@ exports.up = function(knex) {
   })
   .createTable('urls', function(table){
       table.string('shortcode').primary();
-      table.string('target');
+      table.string('target').index();
       table.timestamps();
   });
 };